1. Field of the Invention
The present invention relates to an apparatus for recording signals on an optical disc and reproducing signals from the optical disc, and more particularly to an apparatus for synthesizing signals derived from an optical disc.
2. Description of the Related Art
When an optical disc is used to record and reproduce an information signal, a beam of light is radiated on a recording layer of the optical disc and a returning beam is received by a plurality of light receiving planes (detectors) to obtain a plurality of signals. Each light receiving plane (detector) receives part of the returning beam. The part of the returning beam is referred to as a returning beam segment in this specification. The signals prepared by the light receiving planes are synthesized to create a so-called RF signal used for reproduction of the information signal, a servo signal used for focusing and tracking, or a detection signal used for detection of a pre-pit. A signal synthesizing apparatus is employed when synthesizing the signals returning from the recording layer of the optical disc.
One example of such signal synthesizing apparatus is disclosed in Japanese Patent Kokai (Laid-Open Publication) No. 2000-132835. The signal synthesizing apparatus uses a plurality of light receiving planes to capture a returning light beam. It is therefore ideal that all the light receiving planes have the same light receiving characteristics. In reality, however, it is impossible for the light receiving planes to have the same light receiving characteristics. In order to deal with this problem, the signal synthesizing apparatus of Japanese Patent Kokai No. 2000-132835 relies upon variable gain amplifiers to adjust signal levels of the returning light beam segments such that the signal levels of the returning light beam segments become equal to a predetermined reference value.
Practically, however, a plurality of light receiving elements which constitute each light receiving plane have great variation in optical and physical characteristics. In addition, the reference value mentioned above cannot be separated from the signal levels of the returning light beam segments in order to insure an appropriate functioning of the variable gain amplifiers. This imposes considerable limitations on determination (selection) of the reference value and design of a feedback circuit including the variable gain amplifiers.